1. Field of the Invention
A magneto-rheological damping assembly.
2. Description of the Prior Art
Magneto-rheological (MR) damping assemblies are well known in the art. Such devices are known and used in the automotive field in vehicle suspension systems in the form of shock absorbers, struts, and other motion or vibration damping structures. MR Dampers use magneto-rheological, or MR fluid, which exhibits a thickening behavior (a rheology change) upon being exposed to magnetic fields of a sufficient strength. The higher the magnetic field strength to which the MR fluid is exposed, the higher the viscosity of the fluid, and the higher the damping force of the device.
One such assembly is shown in U.S. Patent Application 2010/0089711 (Hereinafter known as the '711 application). The '711 application discloses a piston extending along an axis defining a compression end and a rebound end. The piston defines a core. An electromagnet is disposed annularly about and engages the core for selectively generating a magnetic flux. The damping force of the assembly is a function of the current supplied to the electromagnet. To provide for desirable level of damping in the absence of a current to the electromagnet and to reduce the required operating current in the device, a permanent magnet is disposed annularly about the electromagnet for generating a magnetic flux. It is known in the art to include a plurality of electromagnets spaced axially from one another. One such assembly is disclosed in U.S. Pat. No. 6,419,057 (Hereinafter referred to as the '057 patent). The '057 patent further discloses a pole segment, or area constructed of a material having a high magnetic permeability for concentrating the magnetic flux from the electromagnets and permanent magnets, disposed axially between permanent magnets.
The '711 application further discloses a main gap extending axially between the compression end and rebound end of the piston and disposed adjacent a pole segment for conveying the MR fluid through the piston. The flux from the magnets changes the viscosity of the fluid in the main gap to control the damping force of the assembly. To control the magnetic flux generated by the electromagnet, the '711 application discloses a controller. The controller defines an off operating state for applying a negative current through the electromagnets for cancelling the flux from the permanent magnets across the main gap to achieve a low damping force.
The '711 application also discloses an auxiliary gap of a material that is less magnetically permeable than the core of the piston to provide for an area of high magnetic reluctance. This is a necessary element of the design because without the auxiliary gap, when it is desirable for flux to travel across the main gap (when the device is not in the off operating state), most of the flux would get short circuited into the core rather than passing across the main gap, leading to an undesirably small damping force.
An identified problem of the prior art is that due to the location of the auxiliary gap, sufficient cancellation of the flux across the main gap is not attainable in the off-operating state due to a leakage of flux from the permanent magnets across the main gap. This is particularly problematic because it results in an undesirably high damping force when the assembly is in the off operating state and it prevents filling of the MR assembly with fluid through the main gap during assembly.